In this work, a constant-pressure model capable to simulate the overlap of chambers in rotary internal combustion engines is proposed. It refers as a chamber overlap when two adjacent chambers are in communication through the same port, which could occur in some rotary internal combustion engines. The proposed model is thermodynamic (or zero-dimensional (0D)) in nature and is designed for application in engine simulators that combine one-dimensional (1D) gasdynamic models with thermodynamic ones. Since the equations of the proposed model depend on the flow direction and on the flow regime, a robust and reliable solution strategy is developed. The model is assessed using a two-dimensional (2D) problem and is applied in the simulation of a rotary internal combustion engine. Results for this last problem are compared with other common approaches used in the simulation of rotary engines, showing the importance of effects such as the interaction between overlapping chambers and the dynamics of the flow.